The US has issued a worldwide travel alert after recent attacks in France, Australia and Canada.
The alert comes hours after French police killed three hostage-takers after three days of terror in Paris.
Two of the men are believed responsible for Wednesday's attack on a satirical magazine Charlie Hebdo in Paris. Twelve people were killed in that assault, France's deadliest terror incident in decades.
The State Department's warning says attacks against Americans are becoming increasingly prevalent.
It also cites an increased risk of reprisals against US and Western targets for the US-led intervention against Islamic State militants in Syria and Iraq.
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The warning also cited last month's deadly hostage standoff in Sydney and the October killing of a soldier near Canada's parliament.
